Physics IA · C · Wave behaviour · topic 18 of 30

Light through a dye: how does absorption change? · IB Physics IA idea: guide, sample graph and step-by-step plan

simple kit, serious analysis. nice.

Add a bit more dye and the light dims. But does it dim in a straight line, or does it curve? Your sensor will tell you.

Difficulty●●○ Medium
KitBasic: lamp or laser pointer, light sensor, clear containers, food dye
TimeAbout 10 hours
FitsSL and HL (the absorption law is beyond the course)
  • ✓ you like measuring with sensors
  • ✓ you can make accurate dilutions
  • ✓ you want a clean exponential to test

The big idea

← the exponential is your hook

Each thin layer of dye absorbs the same fraction of the light reaching it, not the same amount. That predicts exponential decay of transmitted intensity: I = I₀e^(−kx) for a layer of thickness x, where k grows with concentration, so thickness and concentration should play the same role. Your job: vary one thing, keep the light and the sensor identical, and test whether ln(I/I₀) really falls in a straight line. Then decide what the gradient means and where the model breaks (stray light, sensor limits). Clear prediction, clean graph, an evaluation with real numbers.

Make it yours: pick one twist

Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.

Swipe for more twists →

Vary thickness, not concentration

Use one dye solution in containers of different widths, or stack identical coloured filters (gel sheets).

→ Your independent variable is path length. A stack of filters is easy, but each surface also reflects light, so you must think about that.
Use one colour of light

Match the light to the dye: a red laser pointer through a blue dye, or an LED of known colour, instead of a white lamp.

→ Removes the problem that a dye absorbs colours differently. A cleaner exponential, and you can compare two colours.
Test the model both ways

Do a concentration series and a thickness series with the same dye.

→ Tests whether concentration and thickness play the same role (they should, in the simple model). A stronger conclusion.
Find where it fails

Go to high concentrations until the straight line bends.

→ The model becomes something you test at its limit. Think about stray light and the sensor's range.
Compare two dyes or filters

Same method, two dyes, and compare their absorption constants.

→ Your conclusion compares numbers with uncertainties, not just a trend.
Change the context

Sunglasses, window tint or water clarity in a pond or fish tank.

→ Gives your introduction a purpose; the physics and graph stay the same.

Your Physics IA research question: run the check

write it FIRST, then check it

Write your own question first. Then make sure it ticks every box:

  • Names one independent variable (concentration or thickness), its range and how many values.
  • Names the dependent variable and how it is measured (a light sensor reading, in which units, in which set-up).
  • Says what stays fixed: lamp, its distance and warm-up time, the container, the sensor position, dark surroundings.
  • Says what you will compare with: the exponential model, a linear alternative, or both.
  • Is narrow enough for about 10 hours, with 6 or more values and repeats.

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The set-up

One way to arrange it. Draw yours with your real distances and the exact lamp and sensor you used.

xI0Ilamp: warm it up firstsame cuvette (same width x)block room lightsensor: same spot every timelogger

How your Physics IA could be laid out

The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.

Research question and context~250
Your question; where absorption matters (sunglasses, tinted windows, water depth); what you expect and why.
Background physics~450
Absorption as a fraction per layer; the exponential law and its assumptions; what ln(I/I₀) against your variable should look like if it holds.
Variablestable
Independent, dependent and controlled variables, each with how you set it, measure it and why it matters.
Method~400
Apparatus with resolutions; how you make and check each dilution; how you set the light and sensor and shield stray light; how you take I₀; repeats.
Safety, ethics, environment~80
Short and specific: dye handling and spills, any laser pointer (never towards eyes), hot lamps.
Raw datatable
One table: each value of the variable, repeated sensor readings, and the I₀ readings taken during the run.
Processing~250
How you turn sensor readings into I/I₀ and then ln(I/I₀) with uncertainties; one worked example of each calculation type.
Graphs and analysis~350
Which graph and why (a straight line is easier to judge than a curve); error bars; what the gradient means physically; where the data leaves the model.
Conclusion~400
Answer with numbers: is the decay exponential within your uncertainties, what is the constant, and where does the model stop working?
Evaluation~650, the biggest part!
Weaknesses ranked by their effect on your result, each with a specific, realistic improvement.
Referencesnot counted
Every source for theory, method, data or simulation.

What your Physics IA graph could look like

error bars on EVERY point ✓

Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only

01020304050607080901000.00.20.40.60.81.01.2Dye concentration c / % of stock solution (± 1%)Fraction transmitted I/I₀ (± 0.02)exponential modelif absorption were linearyour data, with error barswhich curve do your pointsfollow? Say how you can tellILLUSTRATIVE · MADE-UP DATA
  • Axis labels name the quantity, the unit and the uncertainty of what you set or read.
  • Error bars on every point. Here the sensor uncertainty gives the vertical bars; the dilution gives the horizontal ones.
  • Both models drawn, so the reader can see which one your data follows.
  • The graph shows a fraction, so I₀ is measured on the same day with the same light: say how.
  • Another graph to consider: plot ln(I/I₀) against concentration. If the model holds you get a straight line through the origin, and its gradient is the absorption constant.

Which uncertainty matters most?

Rank by the effect on your result (here, the absorbance −ln(I/I₀) at a transmitted fraction of about 0.33), not on the raw readings. Note how a small stray-light signal matters more as the dye gets darker.

Stray light (1% of I₀)2.7%Dilution volumes (c ± 2%)2%Sensor drift (± 2% of reading)1.8%Cuvette width (± 0.1 mm of 10 mm)1%Lamp drift in I₀ (1%)0.9%biggest effect: discuss this first!% uncertainty in the absorbance at c = 50% of stockILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

Tick steps off as you go. We'll remember on this device.

0 of 11 done

  1. Read and predictFind the exponential attenuation law and what each symbol means. Predict the shape of I/I₀ against your variable before you measure, and say why it should not be a straight line.
  2. Trial run (1 lesson)Does the lamp settle? Does the sensor reading drift? Is room light leaking in? Does the sensor saturate with no dye? Fix these now.
  3. Choose the range and the lightPick 6 or more values from clear to very dark. Decide white lamp, LED or laser, and justify it.
  4. Plan your dilutionsWork out volumes in advance and decide how you will measure them. Write the uncertainty of each one.
  5. Take I₀ properlyDecide what 'no dye' means (empty container, or a container of water) and re-measure it during the run to catch drift.
  6. Collect the dataRandomise the order of values if you can. Record raw readings with units. Note anything odd as it happens.
  7. Process and graphConvert to fractions, propagate uncertainty, and decide which graph tests the model most directly. Add error bars and a best-fit line.
  8. Compare with the modelIs the line straight within your error bars? What does its gradient represent? Where does the data leave the line, and why?
  9. EvaluateRank the uncertainties by effect on your result. Make each improvement specific to a weakness you actually saw.
  10. Write, then cutWrite in the order of the layout above and keep within 3,000 words: tables, graphs, equations and references are not counted, so let them carry detail.
  11. Check before your teacher's draftRun the checklist below, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.

Physics you'll need

Traps to avoid in this Physics IA

check your sensor range!
  • Room light or the lamp's own heat changing the sensor reading, so I₀ is not the same all the way through.
  • Using a white lamp with a coloured dye and then talking as if there were one wavelength.
  • A sensor that saturates or changes range on its own without you noticing: a flat top or a jump in the readings.
  • Dilutions made in a rush: the uncertainty in concentration is never zero.
  • Fitting a straight line to I against c without first checking whether the model predicts one.
  • Ignoring reflections at the container walls when comparing different thicknesses.

More Physics IA ideas like this

Make sure it's yours

This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.

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